1. Field of the Invention
The present invention relates to a substrate processing apparatus for forming a film on a semiconductor substrate, and more particularly, to a substrate processing apparatus for forming a silicon film on a semiconductor substrate to form a solar battery, and a reaction tube for processing a substrate.
2. Description of the Prior Art
Recently, development of alternative energy sources that can replace fossil fuels becomes more important from the point of view of environmental protection, and in the field of solar energy generation which is considered as one of such alternative energy sources, development of low-cost, high-efficiency solar batteries is being accelerated.
In one of solar battery manufacturing processes, a silicon film is formed on a silicon substrate, and as examples of apparatuses configured to form such silicon films, there are a pancake type epitaxial growth apparatus that employs a high-frequency heating method, a barrel type epitaxial growth apparatus, a barrel type epitaxial growth apparatus employing a lamp heating method, and a single wafer type epitaxial growth apparatus.
Such apparatuses are required to perform an epitaxial growth process in a reaction tube at a low pressure state such as a vacuum state and a high temperature state, and thus the reaction tube should have at least good pressure resistance and high temperature resistance. Furthermore, in the case of using a dangerous gas such as H2 gas, a structure having strong internal durability and made of a high-quality material is required to prevent leakage of gas. These requirements increase manufacturing costs of such apparatuses but reduce cost performances of the apparatuses.
Moreover, in the case where a reaction tube is made of quartz and silicon is epitaxially grown in the reaction tube at a low pressure and a temperature of 1000° C. or higher, since the possibility of thermal degradation of the quartz reaction tube is increased, it is preferable that a cooling mechanism be installed at the quartz reaction tube to ensure safety. However, although a cooling mechanism can be simply installed, it is impossible to prevent thermal degradation of the quartz reaction tube completely, and epitaxial substrate processing apparatuses using such quartz reaction tubes having high pressure resistance are expensive. As apparatuses equipped with cooling mechanisms, there already exist a pancake type high-frequency induction heating apparatus on the top of which a water-cooled stainless bell jar is installed, and a lamp heating type epitaxial apparatus in which a quartz bell jar is cooled by a powerful air cooling mechanism. However, since such apparatuses use heating sources having short lifespan such as a high frequency generator or a lamp heating source, there arises a problem of low productivity.
Furthermore, in the case of using monosilane (SiH4) source gas that easily undergoes thermal decomposition, since hot-wall type processing is necessary, a cooling mechanism may not be installed, and moreover, silicon generated by decomposition of the monosilane source gas may easily be attached and deposited onto the inner wall of a reaction tube as well as the surface of a wafer. In the case where a reaction tube is made of quartz, due to the difference between thermal expansion coefficients of a silicon film and the reaction tube, the silicon film may be cracked or stripped to cause, for example, devitrification of the reaction tube, and thus the inner wall of the reaction tube may be seriously damaged and weakened. Therefore, it has been difficult to use such a reaction tube as a production apparatus due to safety concerns. In addition, silicon stripped from the inner wall of a reaction tube contaminates substrates.
In addition, an amorphous silicon film can be formed as a film for an inexpensive solar battery. Therefore, recently, inexpensive large-diameter glass substrates are used for forming thin films of solar batteries, and methods of forming amorphous silicon thin films (which can be formed relatively at a low temperature) on such glass substrates are widely used. However, the use of amorphous silicon thin films results in low solar energy conversion efficiency and limited application fields.
On the other hand, in the case of using single-crystalline and polycrystalline silicon films grown on silicon substrates, the energy conversion efficiency of solar batteries can be increased by a factor of 3 as compared with the case of using amorphous silicon film formed on glass substrates. In addition, the aging variations of the solar batteries may be reduced, and the lifespan of the solar batteries may be increased.
Therefore, if costs of a substrate processing apparatus can be reduced, a solar battery can have many merits in the case of using a silicon epitaxial film formed on a silicon substrate as a single-crystalline silicon film.
In addition, since silicon is the most abundant element on earth, if there is sufficient purification equipment, the costs of power generation with solar batteries may be reduced to levels competitive with the costs of power generation using fossil fuels such as petroleum oil.